Lnutroduction. 3 
converse hypothesis of a warm climate continuous from 
Preglacial times to account for the Iberian plants in the 
west of Ireland and in Cornwall. Either might be true, 
but scarcely both ; for the Irish and Cornish plants are not 
such as could survive a colder climate like that postulated 
by Forbes to explain the migration of the Arctic species. 
We have obtained direct evidence, since Forbes wrote, 
that all Ireland was at one time strongly glaciated, and 
also that Arctic plants once occupied the lowlands of 
Devonshire. 
This problem of the origin of our flora is one which can 
be solved, I think, by the historical method, and that 
seems to be the proper mode of attacking it. No doubt 
the imperfection of the geological record is so great as to 
make the task an exceedingly difficult one; for nowhere 
have we yet discovered a continuous sequence of deposits, 
all fossiliferous, such as would give a connected history of 
our recent animals and plants from their first appearance in 
Britain to the present day. The exact order of succession 
of the deposits, of the physical changes, of the climatic 
alternations, and of the waves of migration, is still uncer- 
tain ; though a definite historical record is gradually being 
built up by the comparison and correlation of numerous 
overlapping chronicles, each recording at most some three 
or four of the subordinate stages or periods. This work of 
correlation, as already mentioned, has been greatly 
facilitated by a detailed examination of extensive areas, 
and a close study of the geology of the more recent deposits. 
In this way I have been enabled to trace the connexion 
between the strata, and often to speak with confidence as to 
the date of groups of fossils which otherwise would have had 
to remain as isolated finds. My own researches have been 
largely aided and supplemented by the examination of 
material obtained from friends working in districts which I 
